How Does Emptying Sand Affect a Balloon's Acceleration?

[Izzybee]

Homework Statement

A balloon with an attached basket and a total mass of 1200 kg is hovering at an altitude of 500m. A bag containing 100kg of sand is emptied over the side of the basket. Find the upward acceleration of the balloon after the sand has been poured out.

Homework Equations

F=ma

The Attempt at a Solution

I have no clue how to start this equation or solve it! Please help!

 

[Kurdt]

The balloon had enough upthrust to keep its own weight in equilibrium. When some of the mass is lost it still has the same upthrust. What then is the acceleration?

 

[Izzybee]

9.8?

 

[Kurdt]

9.8?

What does this mean?

The balloons weight is balanced by the upthrust at first. Therefore the upthrust is equal in magnitude to the balloons weight. What is the balloons weight?